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Thermodynamics I - Enthalpy

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Thermodynamics I - Enthalpy CHEM 2880 - Kinetics Thermodynamics I - Enthalpy Tinoco Chapter 2 Secondary Reference: J.B. Fenn, Engines, Energy and Entropy, Global View Publishing, Pittsburgh, 2003. 1 CHEM 2880 - Kinetics Thermodynamics • An essential foundation for understanding physical and biological sciences. • Relationships and interconversions between various forms of energy (mechanical, electrical, chemical, heat, etc) • An understanding of the maximum efficiency with which we can transform one form of energy into another • Preferred direction by which a system evolves, i.e. will a conversion (reaction) go or not • Understanding of equilibrium • It is not based on the ideas of molecules or atoms. A linkage between these and thermo can be achieved using statistical methods. • It does not tell us about the rate of a process (how fast). Domain of kinetics. 2 CHEM 2880 - Kinetics Surroundings, boundaries, system When considering energy relationships it is important to define your point of reference. 3 CHEM 2880 - Kinetics Types of Systems Open: both mass and Closed: energy can be energy may leave and enter exchanged no matter can enter or leave Isolated: neither mass nor energy can enter or leave. 4 CHEM 2880 - Kinetics Energy Transfer Energy can be transferred between the system and the surroundings as heat (q) or work (w). This leads to a change in the internal energy (E or U) of the system. Heat • the energy transfer that occurs when two bodies at different temperatures come in contact with each other - the hotter body tends to cool while the cooler one warms until thermal equilibrium is achieved (they are both at the same temperature) • heat transfer depends on the heat capacity (C) of the bodies involved - C (J K-1) reflects how much energy is required to heat up an object or substance by 1°C or 1 K (CG = molar heat capacity, J K-1mol-1) Note: the equation q = C)T applies only when C is independent of T, which is usually true over a small )T • sign-convention: heat is positive if it flows into the system (from the surroundings) and negative if it 5 CHEM 2880 - Kinetics flows out of the system (into the surroundings) Work • an energy transfer between system and surroundings that is not heat • any energy transfer that has or could have as its sole effect the raising of a weight (Fenn, p.6) • sign-convention: work is positive if it is done by the surroundings on the system and negative if it is done by the system on the surroundings • can be mechanical, electrical, gravitational etc. • common type discussed in thermodynamics is work of increasing or decreasing volume See Tinoco pp. 18-24 for more examples of work. 6 CHEM 2880 - Kinetics How do you tell if an energy transfer is heat or work? • often depends on how your define the system and surroundings • one method that can differentiate between the two types of energy transfer is considering what happens if a thermal insulator is placed between the system and the surroundings (Fenn, p.6) • if the energy transfers as heat, then this will likely effect the energy transfer • if there is no effect, then the transfer is likely work 7 CHEM 2880 - Kinetics Example: Consider a battery connected to a hotplate heating a water bath. (Fenn, p.7-8) • If the system is defined as the battery alone and the hotplate and water bath are considered part of the surroundings, then the energy transfer is work • surrounding the battery with a thermal insulator would not effect the heating of the water • If the system is defined as the battery and hotplate and the water bath is part of the surroundings, then the energy transfer is heat • placing a thermal insulator between the hotplate and the water bath would effect the heating of the water • If the system is defined as the battery, hotplate and water bath, then no energy transfer takes place (neither work nor heat). There are however some exceptions to this method for distinguishing work and heat. e.g. Radiation • a mirrored surface can reflect electromagnetic radiation, preventing any energy transfer, however this radiation can represent a heat (thermal radiation) or a work (radio waves) energy exchange 8 CHEM 2880 - Kinetics The First Law of Thermodynamics Energy is conserved • Energy can be transferred between the system and surroundings and it can change form, but the total energy of the system plus surroundings remains constant • if the only forms of energy exchanged are q and w • if other forms of energy are involved then terms must be added to this equation for each form of energy • for an isolated system )U = 0 • for any change in a system, )U depends only on the initial and final state and not on the process by which the change occurred Sign Convention work done on the system by the surroundings + work done by the system on the surroundings - heat absorbed by the system from the surroundings + (endothermic) heat absorbed by the surroundings from the system - (exothermic) 9 CHEM 2880 - Kinetics State Variables and Functions • state variables depend only on the state of the system and not how it arrived at that state • e.g. P, V, T, n • generally only a few state variables are required to completely describe a system - other variables can be determined from these few • e.g. for a liquid if P, V, T and chemical composition are specified then density, surface tension, refractive index etc can be determined • q, w, and C (among others) depend on the path taken and are not state variables • state functions depend only on the state of the system and not how it arrived at that state • e.g. U, H, S, G The enthalpy (H) of the system is defined as: 10 CHEM 2880 - Kinetics • changes in state functions depend only on the initial and final states and not on the path taken from one state to another • U (E) and is a state function, q and w are not • in taking a system from an initial state (A) to some new final state (B), a number of different paths are generally available. Although the internal energy change will be the same for all paths, the amounts of heat and work will generally be different for different paths. • e.g. if glucose is combusted or metabolized, the same amount of energy is released • if a system undergoes a long series of steps and returns to its initial state (a cyclic process), the change in U or any other state variable is zero 11 CHEM 2880 - Kinetics Intensive vs Extensive • intensive state properties are independent of the size of the system - they do not change when a system is divided e.g. T, P • extensive state properties are proportional to the size of the system - they do change when a system is divided e.g. mass, V, n, U • an extensive property can be converted to an intensive property by describing it per unit of material (m, n, or V) Extensive Intensive energy (J) energy/mol (J mol-1) heat capacity (J K-1) molar heat capacity(J K-1mol-1) mass (kg) density (kg mol-1) and volume (m3) 12 CHEM 2880 - Kinetics Equations of State • link state variables, most frequently P, V and T Solids and Liquids • solids and liquids don’t change V much with P or T so a first approximation for an equation of state of a solid or liquid is Vconstant • find density at one temperature and you can use that at any temperature to describe the system • liquids and solids do change V slightly with P and T - Tinoco Fig 2.3 shows example for liquid water 13 CHEM 2880 - Kinetics In both plots, the volume of water changes by ~3% over the temperature and pressure ranges shown • the change in V with P at constant T (293 K) is linear • 45.9 x 10-9 atm-1 is the isothermal compressibility of water at 293 K - fractional decrease in V of water for an increase in P of 1 atm • the change in V with T at constant P (1 atm) is not linear, it has a minimum at 277 K 14 CHEM 2880 - Kinetics Gases • V does change significantly with P and T • first approximation is the ideal gas law PV = nRT • this law assumes ideal gas behaviour – gas molecules are perfectly elastic, hard spheres of negligible volume, with no attractive nor repulsive forces between them, moving in a completely chaotic manner with perfectly elastic collisions between molecules and with the walls of the container • the ideal gas law applies to all gases at low pressures • at higher pressures it is a good approximation for most gases, accurate to within ±10% at room temperature and atmospheric pressure • other, more accurate equations of state for gases attempt to account for non-ideal behaviour and contain parameters relating to individual gases • the van der Waals gas equation accounts for attractive forces between molecules (a) and the intrinsic volume of the molecules (b) 15 CHEM 2880 - Kinetics • in this case, changing P from 0 to 1000 atm causes a 1000x decrease in V (compared to 3% decrease for liquid water) 16 CHEM 2880 - Kinetics Mixtures • for mixtures, the amount (m or n) of each component must be specified • for gases the situation is particularly simple as the ideal gas law applies to each component individually and the total pressure is equal to the sum of the partial pressures of each component • liquids and solids are more complicated e.g. the volume of a mixture of liquids is not necessarily equal to the sum of the volumes of the components l 17 CHEM 2880 - Kinetics Paths • changes in state functions are independent of the path taken so it is often convenient to pick a path for which the energy change(s) are easy to calculate • a change from P11, V , T1 to P22, V , T2 might be easiest to calculate considering a series of path steps where one state variable is held constant in each step • first P11, V , T1 to P2, V, T1, then to P22, V , T2 etc.
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